Different types of acoustic devices have been used through the years. One type of device is a microphone. In a microelectromechanical system (MEMS) microphone, a MEMS die includes a diaphragm and a back plate. The MEMS die is supported by a substrate and enclosed by a housing (e.g., a cup or cover with walls). A port may extend through the substrate (for a bottom port device) or through the top of the housing (for a top port device). In any case, sound energy traverses through the port, moves the diaphragm and creates a changing potential of the back plate, which creates an electrical signal. Microphones are deployed in various types of devices such as personal computers or cellular phones.
Differential signals are often desired by users. In one example, a first transducer obtains a first signal and a second transducer obtains a second signal. The signals are subtracted (or added) from each other to produce an output. The subtraction operation removes noise and other undesirable effects from the signals and can produce a larger and stronger signal. However, these approaches require the use of two transducers, which can be expensive and increase the size of the overall device.
As package sizes continue to shrink, it becomes more difficult to maintain high SNR performance within the smaller package size. As mentioned, dual parallel motor designs have been used but these cannot be used for very small package sizes.
The problems of previous approaches have resulted in some user dissatisfaction with these previous approaches.